1. Field of the Invention
The present invention relates generally to a modulation/demodulation device in an orthogonal frequency division multiplexing/code division multiple access (OFDM/CDMA) system, and in particular, to a device for synchronizing a frequency in a time domain in OFDM/CDMA system.
2. Description of the Related Art
In general, an OFDM technique is frequently used in digital transmission systems such as a digital audio broadcasting (DAB) system, a digital television system, a wireless local area network (WLAN), and a wireless asynchronous transfer mode (WATM) system. The OFDM technique is a type of multi-carrier technique which modulates transmission data after dividing, and then transmits the divided modulated data in parallel. The OFDM technique was not widely used for the complex structure. However, the recent progress of various digital signal processing techniques including the fast Fourier transform (FFT) and the inverse FFT (IFFT) has made it possible to utilize the OFDM system. Though similar to the existing FDM system, the OFDM system may have an optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between sub-carriers. Because of the optimal transmission efficiency, the OFDM/TDMA and OFDM/CDMA systems have been proposed for use with the WATM system since it requires high-speed data transmission.
Referring now to FIG. 1, there is shown a block diagram of a general OFDM/CDMA system. A description will now be made regarding a multi-carrier (MC) CDMA system using the OFDM/CDMA technique. The MC-CDMA system includes a transmitter 100 and a receiver 120. The transmitter 100 and the receiver 120 may be equally applied to both the forward link and the reverse link.
With regard to the transmitter 100, a plurality of spreaders 101 spread transmission data using orthogonal codes of length N and PN spreading sequences. Typically, N is 256 in the OFDM/CDMA system. If the transmitter 100 is a forward transmitter, the spreaders 101 include spreaders for user identification and spreaders for base station identification. On the other hand, if the transmitter 100 is a reverse transmitter, the spreaders 101 include spreaders for channel spreading and spreaders for user identification. Herein, the N-bit data will be referred to as chip data. The chip data spread by the spreaders 101 is input to a summer 102 after pilot signal insertion (not shown). The chip data is summed in the summer 102 on a chip unit basis and is output in series to a serial/parallel converter 103. The serial/parallel converter 103 outputs the serial chip data provided from the summer 102 in parallel. At this point, the number of the parallel chip data output can be equal to N or not equal to N. Herein, the number of the parallel chip data is assumed to be N. Further, the parallel sample data is input to an inverse fast Fourier transform (IFFT) device 104. The IFFT device 104 receiving N parallel data samples, performs OFDM modulation on the chip data. In other words, the IFFT device 104 performs IFFT on the chip data, and carries the processed chip data on different sub-carriers having orthogonality in a frequency domain. In the IFFT device 104, the sub-carriers are output in the time domain. The data output from the IFFT device 104 will be defined as sample data, and N data samples will be defined as an OFDM symbol.
The parallel output sample data is input to a parallel/serial converter 105. The parallel/serial converter 105 outputs the same data in series. Further, the parallel/serial converter 105 inserts a guard interval on an N-sample data unit basis, i.e., one-OFDM symbol unit basis. The guard interval is data obtained by copying some sample data at the rear of an OFDM symbol comprised of N data samples, and is inserted at the front of the OFDM symbol. Herein, the data in which a guard interval is inserted on an OFDM symbol unit basis, is defined as an OFDM frame. The length of the guard interval should be set longer than an impulse response length. A transmission filter 106 filters the data output from the parallel/serial converter 105 and transmits the filtered data over a radio channel 107 using an RF (Radio Frequency) module (not shown). The radio channel 107 is an additive white Gaussian channel, so that additive white Gaussian noises are added by an adder 109.
The receiver 120 receives a carrier with the additive white Gaussian noises over the additive white Gaussian channel. The received carrier is converted to a baseband signal through an RF module (not shown). A multiplier 110 compensates for frequency error generated in channel 107 using a frequency correction signal received. An analog to digital converter 115 converts the frequency-corrected analog signal input from the multiplier 110 to digital sample data stream. A serial/parallel converter 111 receives the OFDM symbol in series and outputs N data samples constituting the OFDM symbol in parallel. Though not illustrated, the receiver 120 commonly includes a guard interval remover for removing the guard interval inserted on an OFDM frame unit basis before parallelizing the sample data stream. A fast Fourier transform (FTT) device 112 performs OFDM demodulation on the received sample data carried on the sub-carriers in parallel and converts the respective sub-carriers to the original chip data in the frequency domain. A parallel/serial converter 113 converts the parallel chip data to serial chip data. A despreader 114 despreads the serial chip data input from the parallel/serial converter 113 to restore the original data.
Typically in the OFDM transmission system, if local oscillators in the transmitter and the receiver are not tuned to each other, a frequency offset occurs and causes a loss of orthogonality between the sub-carriers. In this case, even a small frequency offset may cause performance degradation of the receiving system. Therefore, in the OFDM/CDMA WATM transmission system, it is necessary to implement frequency synchronization for maintaining orthogonality between the sub-carriers.
Generally, the frequency synchronization used for a receiver of the OFDM system is performed in two steps, namely, a coarse synchronization and a fine synchronization. The coarse synchronization step removes an initial frequency offset corresponding to multiples of the sub-carrier interval, and the fine synchronization step removes the residual frequency offset remaining after coarse synchronization.
There are two coarse frequency synchronization techniques; one proposed by Classen and Myer, and another by Nogammi and Nagashima.
FIGS. 2 to 4 show a frequency synchronization device for the receiver, using the coarse frequency synchronization technique and the fine frequency synchronization technique.
First, a description will be made regarding the coarse frequency synchronization technique proposed by Classen and Myer, with reference to FIG. 2.
The technique proposed by Classen and Myer uses a test correction frequency, and calculates a correlation between known transmission data and received data while shifting the test correction frequency by a predetermined frequency interval, thereby estimating the frequency offset. This technique uses a property that the correlation value becomes maximum when the test correction frequency is nearest to an actual frequency offset shifted in the actual channel.
Referring to FIG. 2, there is shown a block diagram for detecting the test correction frequency offset. A multiplier 128 compensates for a frequency offset of a received signal using a test correction frequency received. An analog/digital converter (ADC) 129 converts the received analog data to digital data. A guard interval remover 122 removes the guard interval from the received data. A guard interval removing method sets a window having a length of two OFDM symbols and one guard interval, calculates a correlation value while shifting the window by samples, and removes the guard interval beginning at a position where the maximum value starts to be output. An FFT device 124 performs FFT to modulate the sample data output from the multiplier 128, and outputs a chip data stream in common to the despreader, a delay 125 and an estimator 127. The delay 125 delays the chip data for one-chip data length time and then outputs the delayed chip data to the estimator 127. A reference tone pattern generator 126 generates a reference tone having a predetermined pattern known to both the mobile station and the base station, and provides the generated reference tone pattern to the estimator 127.
The estimator 127 outputs an estimated frequency offset {circumflex over (f)}e by receiving the chip data output from the FFT device 124, the delayed chip data output from the delay 125 and the reference tone pattern output from the reference tone generator 126. That is, the estimator 127 outputs the estimated frequency offset {circumflex over (f)}e using a correlation value between the chip data of the two consecutive sub-channels and the reference tone known to the receiver. The estimated frequency offset {circumflex over (f)}e is a factor in determining the test correction frequency.
The estimator 127 calculates the estimated frequency offset in accordance with Equation (1) below.                                           f            ^                    e                =                  MAX          ⁢                      "LeftBracketingBar"                                          ∑                                  k                  =                  1                                L                            ⁢                                                (                                                            Z                                                                        l                          +                          1                                                ,                        k                                                              ·                                          Z                                              l                        ,                        k                                                                              )                                ⁢                                  (                                                            X                                                                        l                          +                          1                                                ,                                                  k                          +                          s                                                                                      ·                                          X                                              l                        ,                                                  k                          +                          s                                                                                                      )                                                      "RightBracketingBar"                                              (        1        )            
where {circumflex over (f)}e denotes the estimated frequency offset, Zl,k and Z1+l,k denote the chip data of the consecutive sub-carriers, Xl,k denotes the data stream previously known to the receiver during data reception, xe2x80x98sxe2x80x99 denotes the frequency shift for sync estimation, xe2x80x981xe2x80x99 denotes an index of the sample data, and xe2x80x98kxe2x80x99 denotes an index of the OFDM symbol. It is noted from Equation (1) that the two consecutive chip data exist in the same OFDM symbol.
Referring to FIG. 3, a description will now be made regarding the coarse frequency synchronization technique proposed by Nogammi and Nagashima. An analog to digital converter (ADC) 131 converts analog data received from a multiplier 140 to digital sample data. A guard interval remover 133 removes from the received data a guard interval which is used for distinguishing the received sample data and for preventing interference between the symbols. An FFT device 135 performs a FFT on the sample data output from the ADC 131, and outputs a chip data stream to both a despreader and a correlator 139. A reference tone pattern generator 137 generates a predetermined reference tone pattern and provides correlator 139 with the reference tone pattern. The correlator 139 outputs an estimated frequency offset {circumflex over (f)}e using the chip data output from the FFT device 135 and the reference tone pattern output from the reference tone pattern generator 137.
The coarse frequency synchronization technique proposed by Nogammi and Nagashima is different from the technique proposed by Classen and Myer in that a correlation value between one data sample and a reference tone known to the receiver is used for frequency synchronization instead of a correlation value between two consecutive data samples and the reference tone.
The estimated frequency offset according to the technique proposed by Nogammi and Nagashima is calculated by Equation (2) below.                                           f            ^                    e                =                  MAX          ⁢                      "LeftBracketingBar"                                          ∑                                  k                  =                  1                                L                            ⁢                              (                                                      Z                                          l                      +                      1                                                        ·                                      X                                          l                      ,                                              k                        +                        s                                                                                            )                                      "RightBracketingBar"                                              (        2        )            
In addition to the coarse frequency synchronization techniques, there are two fine frequency synchronization techniques; one proposed by Dafara and Adami, and another by Moose.
The technique proposed by Dafara and Adami acquires fine frequency synchronization using a property of the transmission signal, namely that when there exists no frequency offset, a signal in the guard interval of the received signal is identical to the original signal. In addition, when there exists a frequency offset, a signal in the guard interval and the original signal have different phases due to the frequency offset, and finally, when the signal in the guard interval is multiplied by the original signal, an imaginary part of the resulting value contains information about the frequency offset. The present invention removes the residual frequency offset according to this property.
Referring to FIG. 4, a description will now be made regarding the fine frequency synchronization technique. A bandpass filter 141 filters analog data and only permits a frequency band that is proper for the system to pass. A multiplier 143 multiplies the filtered received data by the test correction frequency in order to correct a fine frequency offset. An ADC 145 converts the frequency offset-corrected analog data output from multiplier 143 to digital OFDM frame data. A guard interval remover 153 removes the guard interval included in the OFDM frame from the OFDM frame output from the ADC 145, and outputs OFDM symbols. An FFT device 155 parallelizes the OFDM symbols output from the guard interval remover 153 into N data samples, and performs FFT on the N data samples to output N-chip data. In order to simplify the drawing, FFT 155 contains a serial/parallel data converter and a parallel/serial converter similar to elements 111, 112, and 113 of FIG. 1. The data output of FFT 155 is in the for of serial data.
A frequency detector 147 detects a frequency error for compensating for the fine frequency offset. The frequency detector 147 can detect the frequency error through either a path xe2x80x98axe2x80x99 or a path xe2x80x98bxe2x80x99.
The frequency error detection through the path xe2x80x98axe2x80x99 uses the guard interval. More specifically, the frequency detector 147 detects the guard interval from the OFDM frame output from the ADC 145. The detected guard interval is compared with a sample data interval in order to detect the frequency error. The sample data of N sample data of the OFDM symbol used to generate the guard interval out of pure sample data. Specifically, some sample data of N same data are copied and inserted in the beginning of the OFDM symbol.
Frequency error detection through the path xe2x80x98bxe2x80x99 uses the fast Fourier transformed-chip data from FFT 155. For frequency error detection through the path xe2x80x98bxe2x80x99, a carrier extractor 157 is required. The carrier extractor 157 extracts pilot chip data that is inserted in the chip data stream output from the FFT 155 and provides the frequency detector 147 with the extracted pilot chip data. The frequency detector 147 then detects the frequency error by comparing the pilot chip data with a known signal.
The technique proposed by Dafara and Adami uses the xe2x80x98axe2x80x99 path wherein the frequency detector 147 uses the guard interval from the digital data output from the ADC 145 and outputs an estimated fine frequency offset calculated by                                           f            ^                    e                =                              1            L                    ⁢                      xe2x80x83                    ⁢                      tan                          -              1                                ⁢                                    ∑                              k                =                1                            L                        ⁢                                                            Im                  ⁢                  Z                                                  l                  ,                                      N                    -                    1                                                              ·                              Z                                  l                  ,                                      -                    1                                                  *                                                                        (        3        )            
where N denotes the sample number of OFDM symbol, and I denotes the sample number in the guard interval. In addition, m denotes calculating only th imaginary number value of the complex number value and xe2x80x9c*xe2x80x9d refers to the conjugate of a complex number.
The technique proposed by Moose uses the xe2x80x98bxe2x80x99 path wherein the frequency detector 147 receives the pilot signal from the FFT device 155 through the carrier extractor 157 and outputs an estimated fine frequency offset calculated by                                           f            ^                    e                =                              1                          2              ⁢              π              ⁢                              xe2x80x83                            ⁢              r                                ⁢                      xe2x80x83                    ⁢                      tan                          -              1                                ⁢                      {                                                            ∑                                      k                    =                    1                                    L                                ⁢                                  (                                                            Z                                                                        l                          +                          1                                                ,                        k                                                              ·                                          Z                                              l                        ,                        k                                            *                                                        )                                                                              ∑                                      k                    =                    1                                    L                                ⁢                                  (                                                            Z                                                                        l                          +                          1                                                ,                        k                                                              ·                                          Z                                              l                        ,                        k                                            *                                                        )                                                      }                                              (        4        )            
where L denotes the sample number used when estimating the frequency error and r refers to the guard interval.
The estimated fine frequency offset detected by the frequency detector 147 through path xe2x80x98axe2x80x99 or xe2x80x98bxe2x80x99 is input to a voltage controlled oscillator (VCO) 151 through a lowpass filter 149. The voltage controlled oscillator 151 generates the test correction frequency depending upon the estimated fine frequency offset and provides the generated test correction frequency to the multiplier 143.
The guard interval based (GIB) fine frequency synchronizing technique through the path xe2x80x98axe2x80x99 is implemented at a pre-FFT stage, and the pilot signal based fine frequency synchronizing technique (or maximum likelihood estimation (MLE)) through the path xe2x80x98bxe2x80x99 is implemented at a post-FFT stage.
If a test correction frequency offset is {circumflex over (f)}e, then the received baseband signal can be expressed as x(t)ej2xcfx80{circumflex over (f)}et. At this point, in the GIB algorithm, the phase difference between two samples is constantly 2xcfx80{circumflex over (f)}ct. However, in the MLE algorithm, the phase difference between two samples is 2xcfx80{circumflex over (f)}ctxe2x80x2 which is affected by the length of the guard interval.
As described above, the conventional coarse frequency synchronization technique is susceptible to channel noises, therefore, it is difficult to guarantee the system performance.
Further, the fine frequency synchronization is locked at a sub-carrier which is located nearest to a position of the residual frequency offset. However, when the residual frequency offset has a value of about xc2x10.5% of the sub-carrier interval, the conventional fine frequency synchronization technique cannot acquire synchronization.
Moreover, the MLE algorithm processes the data at the post-FFT stage, thereby causing a delay in acquiring synchronization. This delay will increase the time required for synchronization acquisition.
It is therefore, an object of the present invention to provide a frequency synchronizing device for performing frequency synchronization using only the guard interval signal in a time domain in an OFDM/CDMA system.
It is another object of the present invention to provide a frequency synchronizing device for acquiring accurate synchronization by performing frequency synchronization in the steps of coarse, regular and fine frequency synchronization in a time domain in an OFDM/CDMA system.
To achieve the above objects, there is provided a frequency synchronizing device for an OFDM/CDMA communication system which exchanges data using an OFDM frame including OFDM symbols each comprised of a plurality of data samples, and a guard interval inserted at the head of each symbol to prevent interference between the symbols. The frequency synchronizing device comprises a frequency corrector for compensating for a frequency offset of the received analog data according to a frequency correction signal; an analog/digital converter for converting the received analog data to an OFDM frame; and a frequency synchronizer for detecting copy data which is used for creating the guard interval from the OFDM frame. The copy data is comprised of some data samples out of the OFDM symbols and is used to sequentially estimate coarse, regular and fine frequency offsets, and provide the frequency corrector with the frequency correction signal corresponding to the estimated frequency offsets. The copy data is equal to the guard interval within an OFDM frame.